Re: [Qemu-devel] [PATCH v6 12/20] trace: don't abort qemu if ftrace can't be initialized

[Eric Blake]

On 10/04/2016 08:35 AM, Daniel P. Berrange wrote:
> If the ftrace backend is compiled into QEMU, any attempt
> to start QEMU while non-root will fail due to the
> inability to open /sys/kernel/debug/tracing/tracing_on.
> 
> Add a fallback into the code so that it connects up the
> trace_marker_fd variable to /dev/null when setting

s/setting/getting/ ?

> EACCESS on the 'trace_on' file. This allows QEMU to

s/EACCESS/EACCES/

> run, with ftrace turned into a no-op.
> 
> Signed-off-by: Daniel P. Berrange <address@hidden>
> ---
>  trace/ftrace.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/trace/ftrace.c b/trace/ftrace.c
> index e953922..3588bb0 100644
> --- a/trace/ftrace.c
> +++ b/trace/ftrace.c
> @@ -51,6 +51,12 @@ bool ftrace_init(void)
>          snprintf(path, PATH_MAX, "%s/tracing/tracing_on", debugfs);

Unrelated, but it would be nice to get rid of the PATH_MAX stack
allocation, and instead use g_strdup_printf() or similar in a followup
patch.

>          trace_fd = open(path, O_WRONLY);
>          if (trace_fd < 0) {
> +            if (errno == EACCES) {
> +                trace_marker_fd = open("/dev/null", O_WRONLY);
> +                if (trace_marker_fd != -1) {
> +                    return true;
> +                }
> +            }
>              perror("Could not open ftrace 'tracing_on' file");
>              return false;
>          } else {
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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